| L-dopa |
| A chemical related to dopamine that is used in the treatment of Parkinson's disease. |
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| labia majora |
| The outer folds of skin that cover and protect the genital region in women. |
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| labia minora |
| Thin membranous folds of skin outside the vaginal opening. |
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| lactose intolerance |
| A genetic trait characterized by the absence of the enzyme lactase, which breaks down lactose, the main sugar in milk and other dairy products. |
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| Langerhans' cells |
| Epidermal cells that participate in the inþammatory response by engulfing microorganisms and releasing chemicals that mobilize immune system cells. |
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| large intestine |
| Consists of the cecum, appendix, colon, and rectum; absorbs some nutrients, but mainly prepares feces for elimination. |
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| larva |
| A stage in the development of many insects and other organisms including sea urchins and sponges. In sponges, sexual reproduction results in the production of motile ciliated larvae. |
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| larynx |
| A hollow structure at the beginning of the trachea. The vocal cords extend across the opening of the larynx. |
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| lateral roots |
| Roots extending away from the main (or taproot) root. |
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| latitudinal diversity gradient |
| The decrease in species richness that occurs as one moves away from the equator. |
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| latitudinal gradient |
| As latitude increases, a gradient of cooler, drier conditions occurs. |
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| law of the minimum |
| Holds that population growth is limited by the resource in shortest supply. |
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| leaf primordia |
| Young leaves, recently formed by the shoot apical meristem, located at the tip of a shoot. |
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| leaf veins |
| Vascular tissue in leaves, arranged in a net-like network (reticulate vennation) in dicots, and running parallel (parallel vennation) to each other in monocots. |
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| leaves |
| The site of photosynthesis; one of the three major organs in plants. |
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| leukemia |
| Cancer of the developing blood cells in the bone marrow. Leukemia leads to rampant overproduction of white blood cells (leukocytes); symptoms usually include anemia, fever, enlarged liver, spleen, and/or lymph nodes.
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| leukocytes |
| White blood cells; primarily engaged in fighting infection. |
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| library |
| A collection of cloned DNA, usually from a specific organism.
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| lichens |
| Autotrophic organisms composed of a fungus (sac or club fungus) and a photosynthetic unicellular organism (e.g., a cyanobacterium or alga) in a symbiotic relationship; are resistant to extremes of cold and drought and can grow in marginal areas such as Arctic tundra. |
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| life history |
| The age at sexual maturity, age at death, and age at other events in an individual's lifetime that inþuence reproductive traits. |
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| ligaments |
| Dense parallel bundles of connective tissue that strengthen joints and hold the bones in place. |
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| light reactions |
| The photosynthetic process in which solar energy is harvested and transferred into the chemical bonds of ATP; can occur only in light. |
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| lignin |
| A polymer in the secondary cell wall of woody plant cells that helps to strengthen and stiffen the wall; related term lignified. |
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| linkage |
| The association of genes and/or markers that lie near each other on a chromosome. Linked genes and markers tend to be inherited together.
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| lipases |
| Enzymes secreted by the pancreas that are active in the digestion of fats. |
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| lithosphere |
| The solid outer layer of the Earth; includes both the land area and the land beneath the oceans and other water bodies. |
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| lobe-finned |
| Fish with muscular fins containing large jointed bones that attach to the body; one of the two main types of bony fish. |
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| locus |
| The place on a chromosome where a specific gene is located, a kind of address for the gene. The plural is "loci," not "locuses."
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| LOD score |
| A statistical estimate of whether two loci are likely to lie near each other on a chromosome and are therefore likely to be inherited together. A LOD score of three or more is generally taken to indicate that the two loci are close.
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| logistic growth model |
| A model of population growth in which the population initially grows at an exponential rate until it is limited by some factor; then, the population enters a slower growth phase and eventually stabilizes. |
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| long-day plants |
| Plants that þower in the summer when nights are short and days are long; e.g., spinach and wheat. |
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| loop of Henle |
| A U-shaped loop between the proximal and distal tubules in the kidney. |
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| lungfish |
| A type of lobe-finned fish that breathe by a modified swim bladder (or lung) as well as by gills. |
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| lungs |
| Sac-like structures of varying complexity where blood and air exchange oxygen and carbon dioxide; connected to the outside by a series of tubes and a small opening. In humans, the lungs are situated in the thoracic cavity and consist of the internal airways, the alveoli, the pulmonary circulatory vessels, and elastic connective tissues. |
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| luteal phase |
| The second half of the ovarian cycle when the corpus luteum is formed; occurs after ovulation. |
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| luteinizing hormone (LH) |
| A hormone secreted by the anterior pituitary gland that stimulates the secretion of testosterone in men and estrogen in women. |
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| lymph |
| Interstitial þuid in the lymphatic system. |
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| lymph hearts |
| Contractile enlargements of vessels that pump lymph back into the veins; found in fish, amphibians, and reptiles. |
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| lymphatic circulation |
| A secondary circulatory system that collects þuids from between the cells and returns it to the main circulatory system; the circulation of the lymphatic system, which is part of the immune system. |
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| lymphatic system |
| A network of glands and vessels that drain interstitial þuid from body tissues and return it to the circulatory system. |
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| lymphocyte |
| A small white blood cell that plays a major role in defending the body against disease. There are two main types of lymphocytes: B cells, which make antibodies that attack bacteria and toxins, and T cells, which attack body cells themselves when they have been taken over by viruses or become cancerous.
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| lymphocytes |
| White blood cells that arise in the bone marrow and mediate the immune response; include T cells and B cells. |
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| lysosomes |
| Membrane-enclosed organelles containing digestive enzymes. The lysosomes fuse with food vacuoles and enzymes contained within the lysosome chemically breakdown and/or digest the food vacuole's contents. |
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